Conventionally, there are a plurality of methods of manufacturing the casting mold. As to one of the method of manufacturing the casting mold, the shell molding method is well known. The shell molding method is a method for manufacturing the casting mold by bonding the refractory aggregates by the binder. The refractory aggregates are exemplified by silica sand. The shell molding method is realized as an effective method for producing the casting mold having a good dimensional accuracy. According to this reason, the shell molding method is frequently used when producing the casting mold.
As to the binder for the shell molding method, the thermosetting resin such as phenol resin is generally used. The refractory aggregates are mixed with the thermosetting resin to produce the resin coated sand. The resin coated sand comprises the refractory aggregates each are coated at its surface with the thermosetting resin. According to filling the metallic mold with the resin coated sand and then melting-curing the thermosetting resin, the casting mold is produced.
The resin coated sand is granulated substance which itself is dry. In addition, the resin coated sand has a good flowability. Therefore, it is possible to easily fill the metallic mold with the resin coated sand. In addition, the resin coated sand has a good deterioration resistance when handling is made under air atmosphere. According to this, the shell molding method using the resin coated sand is well-used for manufacturing the mold for the automobile.
In the resin coated sand, as to the thermosetting resin binder which coats the surface of the refractory aggregate, as explained above, the phenol resin is conventionally used. Especially, a novolac-type phenolic resin is well-used as the thermosetting resin binder. The novolac-type phenolic resin has a thermoplastic property. The novolac-type phenolic resin is not cured when heated. Therefore, in using the novolac-type phenolic resin, a curing agent is typically mixed with the novolac-type phenolic resin. The curing agent is exemplified by the hexamethylenetetramine, (Refer to Patent Literature 1 hereinafter explained)
When the resin coated sand having the novolac-type phenolic resin as the binder is filled in the heated metallic mold having a temperature of, for example, 250 degrees Celsius to 350 degrees Celsius, the heat in the metallic mold is transferred to the hexamethylenetetramine. Then, the hexamethylenetetramine is decomposed into the formaldehyde and the ammonia. A most part of the formaldehyde reacts with the novolac-type phenolic resin. According to this reaction, the phenol resin is cured to have a non-solubility.
However, “a part of the formaldehyde which is not reacted” and “most of ammonia” are volatilized into the atmosphere during and after producing the casting mold. The formaldehyde and the ammonia cause the environmental pollution. When the resol-type phenolic resin is used as the phenol resin, it is possible to reduce the above problem. However, also in using the resol-type phenolic resin, the some formaldehyde is unreacted and is volatilized into the atmosphere. Therefore, the resol-type phenolic resin has the above problem, too.
In addition, when the resin coated sand with the phenol resin as the binder is used in manufacturing of the molding, the molten metal is poured into the casting mold. However, when pouring the molten metal into the casting mold, the heat in the molten metal degrades the phenol resin. Consequently, the gases such as phenol, xylenol, toluene, benzene, and methane are released. This also causes the problem of pollution.
Furthermore, the phenol resin has a good heat resistance property. In addition, the phenol resin has a large amount of residual carbon. Therefore, even if pouring the molten metal into the casting mold to form the cast metal is performed, it is difficult to break the casting mold by the heat of the molten metal. Owing to this, in order to demold the cast metal from the casting mold, there is a need to apply the force to the casting mold. In addition, there is a need to apply the high-temperature heat over a long period to decompose the phenol resin. That is, demolding the cast metal from the casting mold requires the additional work.